Site 13: Geology

Here we are at the last stop to view the beautiful Olympic Mountain Range. Does anyone know what formed these peaks? (field answers) So it all has to do with plate tectonics. In fact, the interactions among the plates that compose the Earth’s crust are the reason for both the Olympic and the Cascade mountain ranges. Washington sits on the edge of a plate called the North American plate. Another plate called the Juan de Fuca plate that lies under the Pacific Ocean to the west of us is being pushed towards us, and in the process is sliding under the North American plate. This is called a subduction zone. This sliding also explains the historic earthquakes in our area.

                The Olympic mountain range was formed as the basalt rock of the Juan de Fuca plate was scraped off the top of the plate as it was subducted under the North American Plate. In fact, you can find old ocean sediments filled with shells in the Olympics. The Cascade range is another testament to the consequences of a subduction zone. As the Juan de Fuca plate sinks under the North American plate, it begins melting back into magma. Some of this magma rises up through cracks in the North American plate and, over time, have formed this range which includes three volcanoes. Can you guess how many of these are active or inactive? (field answers) In fact, all three of them are active! These include Mount Baker at the northern tip of the Cascades, Glacier Peak, and Mount Rainier.

                The mountains provide not only spectacular scenery, but can add so much to the amazing diversity of habitat and species we are privileged to see here in the Pacific Northwest!

Olympic Mountain Range

Washington's Subduction Zone
Photo: http://www.dailykos.com/story/2013/02/28/1188893/-Tsunami-Geology-and-the-Quileute-Nation#